Agriculture Reference
In-Depth Information
gas (equivalent to 8.5 m
3
of air at equal
temperature and pressure) (ASAE, 1988). In
practice, 60% surplus of air is supplied (air
factor of 1.6).
In natural gas boilers the heat of the
combustion gases can be recovered as well
as the CO
2
. Natural gas, such as propane and
butane, almost don't have problems of
harmful gases (NO
x
and SO
2
). A complete
combustion must be achieved, to avoid the
formation of toxic CO (carbon monoxide)
and other gases such as ethylene and pro-
pylene. It is very important (and conven-
ient) to install a CO analyser/monitor. When
the time of heating and CO
2
supply do not
coincide, the hot water is stored.
Other devices used are CO
2
generators
that produce CO
2
by combustion (these are
in fact open-flame natural gas burners).
Their cost is low and they only produce a
little heat but they increase the RH of the air
(Photo 9.2). As a guide, propane supplies
three volumes of CO
2
and four volumes of
water vapour per volume of burned gas;
butane supplies four volumes of CO
2
and
five volumes of water vapour per volume of
gas burned; whereas natural gas supplies
one volume of CO
2
and two volumes of
water per volume of natural gas burned
(ASAE, 2002).
Biogas coming from fermentation gen-
erates CO
2
which can be used for carbon
enrichment. If the biogas comes from anaer-
obic fermentation, combustion would be
used first to remove the methane (Urban,
1997a).
9.2.3
CO
2
enrichment techniques
There are two main CO
2
supply sources: (i)
supply in the form of pure gas; and (ii) sup-
ply of CO
2
generated by burning organic
substances. The generation of CO
2
by
decomposition of organic matter, which was
important in the past and is not used nowa-
days, may be considered as a form of that
produced by combustion. Carbonic ice
(solid CO
2
) has only been used in the labora-
tory, due to its high cost and because it nota-
bly decreases the temperature.
Soil mulching was used in the past to
increase CO
2
levels, activating the decom-
position of the organic matter of the soil
(Levanon
et al
., 1986). In a sand-mulched
soil, just after the supply of organic matter,
the decomposition of the organic matter
during the first months may involve a rele-
vant supply of CO
2
to the crop.
Pure CO
2
This is the ideal method, as it can be applied
at any time and in any desired amount, only
being limited by the capacity of the equip-
ment. Unfortunately, the cost of pure CO
2
is
much higher than that from other sources.
In the gas phase CO
2
is colourless, odourless
and incombustible. It is heavier than the air
(density 1.52 kg m
−3
, at normal temperature
and pressure).
It is supplied in small bottles or from a
central tank with a distribution network.
The CO
2
is stored as a liquid at low tempera-
ture and under pressure.
When exact control of the input as a
function of the CO
2
levels in the air is not
possible, some authors recommend to add
5.6 g m
−2
h
−1
(Hicklenton, 1988).
Enrichment with small burners
The main use of burners that send the com-
bustion gases directly inside the greenhouse
may be the supply of CO
2
or the supply of
CO
2
and heating simultaneously. In the first
case, the equipment has lower capacity.
In relation to the air supply, there are
two types of burners: (i) without a fan; and
(ii) with a fan. In all of them, the air supply
is critical for good combustion. Besides, if
the burner is used often and there is no
renewal of the air (ventilation) the lower
oxygen level may result in incomplete com-
bustion. Therefore, it is usual to increase
Combustion gases
The gases must be devoid of harmful com-
ponents, so the fuels, such as natural gas,
paraffin or propane, must have a low sulfur
content.
Natural gas is the most used. It pro-
duces 1.8 kg CO
2
m
−3
of gas, at 20°C and
standard atmospheric pressure and its com-
bustion requires 1.77 m
3
of oxygen m
−3
of
